Fluidized bed boilers comprise a combustion chamber where the fuel is introduced and suspended by an ascending current of cumbustion-supporting air entraining a more or less considerable portion of ash which it is advantageous to recycle to facilitate the regulation of the temperature of the bed and/or to ensure complete combustion. The recovery of the ash is generally effected before the change bundle by convection, with the purpose of protecting the latter from erosion phenomena. In this case the trapping of the ash is customarily ensured by dust-removal equipment not integral with the boiler. The design of these boilers is cumbersome and leads to the use of relatively long ducts.